1. Field of the Invention
The invention relates to a charged particle beam device using a retarding field analyzer to inspect specimens with a primary charged particle beam. The invention also relates to a retarding field analyzer that can be used in devices like a charged particle beam device.
2. Description of the Related Art
Retarding field analyzers in charged particle beam devices are frequently used to analyze the energy distributions of secondary charged particles that are generated by a primary charged particle beam on a specimen. Since the energy distributions of the secondary charged particles carry information about the electrical potential and material of the specimen in the region where the primary charged particle beam interacts with the specimen, a scanning charged particle beam device with a retarding field analyzer can be used to generate a map with voltage or material distributions on the surface of the specimen. Such measurements are usually referred to as voltage- or material contrast measurements.
Voltage and material contrast measurements of specimens by means of charged particle beams have evolved into a highly sophisticated technique. For example, the review article of E. Menzel and E. Kubalek “Fundamentals of Electron Beam Testing of Integrated Circuits”, in SCANNING Vol. 5, 103-122 (1983) describes the use of retarding field analyzers in scanning electron microscopes to measure voltage- and material contrast of integrated circuits with a spatial resolution in the sub-micron range. More recent descriptions on voltage contrast measurements are given in the article “Electrical testing for failure analysis: E-beam Testing” by Michel Vallet et al. in Microelectronic Engineering 49 (1999) p. 157-167, and in “Microanalysis Using Secondary Electrons in Scanning Electron Microscopy” by S. Mil'shtein et al. in Scanning vol. 23, p. 295-297 (2001).
Retarding field analyzers are used to discriminate charged particles according to their kinetic energy with high energy resolution. This is achieved by providing a well-defined electrical potential barrier which rejects charged particles with an energy too low to overcome the potential barrier. Charged particles that have a sufficient energy, however, overcome the potential barrier and are detected by a charged particle detector.
The discrimination of charged particles according to their energy by means of a retarding field analyzer is usually used to reconstruct an energy distribution of the incoming charged particles. An energy distribution of the incoming electrons with the retarding field analyzer is obtained by measuring the detection rates for a set of predetermined electrical potential barriers of various potential energies. If an energy distribution measurement with a high energy resolution is required, the energy intervals between the various electrical potential barriers need to be small. This in turn requires the electrical potential barriers to be well defined.
FIG. 1a and FIG. 1b illustrate the working principle of a planar retarding field analyzer 1 for analyzing the energy distribution of incoming electrons 2. The retarding field analyzer 1 comprises an entrance grid electrode 10 at a second voltage V2 at Z-position P1, a filter grid electrode 4 at a first voltage V1 at Z-position P2 and an electron detector 8 at a detector voltage VS at Z-position P3. The region between the entrance grid electrode 10 and the filter grid electrode 4 is referred to as the retarding electric field region 20. The first voltage V1 is more negative than the second voltage V2 in order to provide a retarding electric field 6 within the retarding electric field region 20 that decelerates incoming electrons 2 that have passed through the entrance grid electrode 10. In the addition, entrance grid electrode 10 and the filter grid electrode 4 are coplanar to each other in order to provide equipotential lines 14 coplanar to the entrance grid electrode 10. With the parallel equipotential lines 14, incoming electrons that enter the retarding field region 20 at a vertical entrance angle 30 with respect to the entrance grid electrode 10, do not experience a lateral force by the retarding electric field 6 and therefore do not change direction.
Incoming electrons 2b that have an energy higher than the potential barrier height 28 (see FIG. 1b), which is given by Ep=e(V2−V1), and enter the retarding field region 20 at a vertical entrance angle 30, overcome the electric potential barrier 26 and pass through the filter grid electrode 4 to enter the accelerating electric field region 22 with the accelerating electric field 12. Incoming electrons 2a with energy too low to summount the potential barrier height 28 do not enter the retarding field region 20. The accelerating electric field 12 serves to accelerate the electrons in order to increase the detection efficiency of the electron detector 8. The electron detector 8 in turn converts the electron signal into a current J which is measured by the current measurement device 24.
It is important for a precise voltage and material contrast measurement that the retarding field analyzer discriminates the incoming electrons according to the same energy and independent of the position of where the electrons 2 enter the retarding field analyzer. To achieve this, it is important that the electrons 2 do not experience lateral forces in the retarding electric field region 20. This implies that the equipotential lines 14 in the retarding electric field region 20 are coplanar to the entrance grid electrode 10 and the filter grid electrode 4, which in turn implies that the entrance grid electrode 10 and the filter grid electrode 4 should be as coplanar as possible to each other.
FIG. 2a and FIG. 2b illustrate the working principle of a spherical retarding field analyzer 50 analyzing the energy distributions of incoming electrons 2. Its operation is like the one of a planar retarding field analyzer 1. However, for the spherical retarding electric field 50, the spherical entrance grid electrode 10 and the spherical filter grid electrode 4 are concentrically arranged to each other in order to provide a retarding electric field region 20 with equipotential lines 14 that run concentrically to the entrance grid electrode 10. Ideally, the concentrically arranged equipotential lines 14 make sure that incoming electrons 2 that enter the entrance grid electrode 10 at a vertical entrance angle 30 do not experience a lateral force due to the retarding electric field 6. Instead, like in FIGS. 1a and 1b, they should be decelerated without changing direction. Provided that there is a vertical entrance angle 30 and no deflection in the openings of the entrance grid electrode 10, the incoming electrons 2 experience the same energy cut, given by Ep=(V2−V1), which is independent of the position where they enter the retarding electric field region 20. Spherical retarding field analyzers are usually employed when the incoming secondary charged particles approach the entrance grid electrode 10 not in parallel but in diverging directions with a significant divergence angle.
However, problems arise when the entrance grid electrode and the filter grid electrode are not coplanar or not concentric. In this case, the equipotential lines 14 are not homogeneously distributed but distorted. The same is true for regions at the edges of the entrance grid electrode and the filter grid electrode. Regions with distorted equipotential lines are called stray field regions.
Incoming electrons which enter a retarding electric field region within the stray field region experience a lateral field force and change direction. Further, electrons that have changed direction need a higher energy to overcome a given potential barrier than electrons that enter the retarding electric field region in a coplanar field region in the direction of the electric field. In other words, the potential barrier height 28 depends on the position at which the electrons pass through the entrance grid electrode 10. As a consequence, the potential barrier height 28 is blurred as indicated in FIG. 3 by the two potential barriers 28c, effective for electrons entering the retarding electric field in the coplanar field region, and potential barriers 28d, effective for electrons entering the retarding electric field in a stray field region. A blurred potential barrier height diminishes the ability of the retarding field analyzer to distinguish between different energy distributions.
In recent years, low energy (e.g. 100 eV to 2000 eV) electron beam microscopes have been developed for the inspection of specimens with high spatial resolution (smaller than 10 nm) and high throughput. Such a device is described for example in the publication of J. Frosien, S. Lanio, H. P. Feuerbaum in “High precision electron optical system for absolute CD-measurements on large substrates” in: Nuclear Instruments and Methods in Physics Research A 363 (1995) 25-30. High spatial resolution at high throughput is achieved e.g. by using a combined electrostatic magnetic objective lens, a high voltage beam tube and in-lens detectors, all three of which are shown in FIG. 4.
FIG. 4 schematically illustrates an electron beam microscope 100 with a combined electrostatic magnetic objective lens 123, a high voltage beam tube 107 and an in-lens detector 114. The primary electron beam 104 that probes the specimen 102 is generated at the electron beam source 106 with a voltage Vcath and is focused with focusing units 120. The primary electron beam 104 is accelerated by means of the beam anode 101 having an anode voltage Vanode. For this type of microscope, the anode voltage Vanode is also applied to the high voltage beam tube 107 which guides the primary electron beam 104 at a high energy to the combined electrostatic magnetic objective lens 123. The combined electrostatic magnetic objective lens 123 is comprised of the magnetic objective lens activated by the magnetic coil 121 and the electrostatic objective lens comprised of the electrostatic electrodes 110, 112 of the beam column 109 and of the high voltage beam tube 107, respectively. The combined electrostatic magnetic objective lens 123 decelerates the primary electron beam 104 and focuses it onto the specimen 102 at the focus position 126. The field of the electrostatic objective lens can be adjusted by changing the voltage of electrode 110 independently of beam column 109.
The high voltage beam tube 107 serves to guide the primary electron beam 104 at a high energy close towards the specimen 102 before the primary electrons are decelerated by electrostatic objective lens 110, 112 and the potential Vsp of specimen 102. The high voltage of the high voltage beam tube, which typically is in the range of 2,000V to 10,000 V, reduces beam spread during the electron transport from the electron beam source 106 to the specimen 102 and allows for higher beam currents. The high voltage beam tube 107 therefore facilitates the operation at high current beams and high spatial resolution for low electron energy beam inspection of the specimen 102.
The electron beam microscope 100 of FIG. 4 is further characterized by its in-lens detector design. In-lens detector design refers to the fact that detector 114 and objective lens 123 are designed in a way that the detector 114 detects secondary charged particles 105 that pass through the aperture of the combined electrostatic magnetic objective lens 123. This allows for the detection of secondary charged particles 105 that leave the specimen 102 in a direction close to the opposite direction of the primary electron beam 104. Further, the secondary particles 105 that arrive at the in-lens detector 114 have been accelerated to a high energy due to the high voltage of the high voltage beam tube 107.
Not shown in FIG. 4 is the “crossover region” of the beam of secondary charged particles 105. The crossover region is an area where the emitted secondary charged particles 105 form a crossover due to the electric and magnetic fields of the combined electrostatic magnetic optical lens 126. The position and shape of the crossover also determine the direction of the secondary charged particles after their passage through the crossover region.
The detector 114 in FIG. 4 typically comprises a scintillator to convert the electron signal into light and a light guide to transport the light to a photomultiplier. The photomultiplier in turn delivers the signal to an electronic device that registers the signal for evaluation.
For many applications, in particular for measuring voltage and/or material contrast of specimens, it would be advantageous to provide a charged particle beam device with a retarding field analyzer with high energy resolution, large acceptance and precise voltage and/or material contrast sensitivity. However, high energy resolution requires a retarding field analyzer with a small ratio of the sizes of the stray field region 41 compared to the size of the coplanar (or concentric) field region 40. However, providing a small stray field region ratio is difficult to achieve if the space for the retarding electric field region within the charged particle beam device is small.
It would be further advantageous to place the retarding field analyzer near the primary charged particle beam in order to detect secondary charged particles that are emitted from the specimen in a direction opposite to the primary charged particle beam. This implies that the retarding field analyzer is placed near or within the high voltage beam tube. However, it is difficult to design a retarding field analyzer with a small stray field region if the retarding field analyzer is placed near or within the high voltage beam tube, since the high voltage intrudes into the retarding electric field and increases the stray field region.
Vice versa, the electric fields of the retarding field analyzer will also disturb the primary charged particle beam in regions where the charged particle beam is not shielded. The larger the potential difference between the high voltage beam tube and the filter grid electrode, the larger the disturbance on the charged particle beam. Therefore, it would further be advantageous to provide a retarding field analyzer that does not influence the primary charged particle beam.
Further, the retarding field analyzer of a charged particle beam device with a high-voltage beam column and in-lens detector design detects secondary charged particles which are accelerated to high energies. On the other hand, the energy range for voltage or material contrast measurements is only a few volts and for some applications, a voltage resolution of less than 10 mV is required. Therefore, it represents a considerable problem for the retarding field analyzer to discriminate secondary charged particles within an energy range of a few eV or less while the mean energy of the secondary charged particles is in the range of several keV.
It is therefore an object of the present invention to provide a retarding field analyzer that overcomes the problems mentioned above.
It is further an object of the present invention to provide a charged particle beam device that can be operated with a retarding field analyzer in the vicinity of the primary charged particle beam without disturbing the primary charged particle beam.
It is further an object of the present invention to provide a retarding field analyzer that can be operated with high energy resolution even when located in the vicinity of a beam tube element of a charged particle beam device.